deprecate support for using Lmod 6.x

[boegel]

The last EasyBuild user survey (conducted in Dec 2018) indicated that about 25% of the EasyBuild community was using Lmod 6 at the time. The most recent release of Lmod 6.x was in November 2016, so it's fair to assume that most people have upgraded to Lmod 7 or 8 by now.

We won't actively break support for Lmod 6 for now, but we need to trim down on the test configurations we're using in Travis CI & GitHub Actions to avoid waiting forever on test results.

With this change included, we will:

• only test with Lmod 6 (& Python 2.7) in a single test configuration in Travis CI (and no more in GitHub Actions)
• produce a deprecation warning when Lmod 6 is used:

$ eb example.eb

WARNING: Deprecated functionality, will no longer work in v5.0:
Support for Lmod version < 7.0.0 is deprecated, found version 6.6.3;
see http://easybuild.readthedocs.org/en/latest/Deprecated-functionality.html for more information

If desired, this warning can be avoided by configuring EasyBuild to skip the modules tool version check via --disable-modules-tool-version-check or by setting $EASYBUILD_DISABLE_MODULES_TOOL_VERSION_CHECK=1 (which is, for obvious reasons, not recommended).

In addition, I've updated some of the modules tool log messages to clearly mention which modules tool is being used (see the NAME class variable that was set for each type of modules tool).